1. Field of the Invention
The present invention relates to an electric storage device including a nonaqueous electrolyte solution in which an electrolyte is dissolved in a nonaqueous solvent.
2. Background Art
In recent years, rechargeable electric storage devices including nonaqueous electrolyte cells represented by lithium ion cells and capacitors such as electric double layer capacitors and the like have been used as power sources for electronic equipment, power sources for power storage, power sources for electric vehicles and the like, whose performance enhancement and downsizing have been progressed.
The nonaqueous electrolyte cell is a cell in which a negative electrode and a positive electrode prepared by providing a negative electrode active material layer and a positive electrode active material layer on a current collector composed of a metal foil, respectively, are arranged to face each other through a separator electrically separating the electrodes, and ions are accepted and donated between the positive electrode and negative electrode in a nonaqueous electrolyte solution, in which an electrolyte is dissolved in a nonaqueous solvent, to charge and discharge the cell.
Known examples of the nonaqueous electrolyte solution include solutions that contain a lithium phosphate derivative such as lithium difluorobis(oxalato)phosphate (Patent Document 1: Japanese Patent No. 3974012 and Patent Document 2: Japanese Patent Application Laid-Open No. 2007-335143).
Adding lithium difluorobis(oxalato)phosphate to a nonaqueous electrolyte solution increases the cycle performance of the cell and enhances the high-temperature storageability of the cell. Accordingly, the nonaqueous electrolyte solution to which lithium difluorobis(oxalato)phosphate is added can have great effects as the nonaqueous electrolyte solution for nonaqueous electrolyte cells such as lithium ion cells. The greater the additive amount of lithium difluorobis(oxalato)phosphate, the greater the effect is.
However, lithium difluorobis(oxalato)phosphate added to the nonaqueous electrolyte solution of a cell can be reductively decomposed to generate gas during initial charge. As a result, the cell can swell. When gas is generated between the negative and positive electrodes, current applied concentrates on that location, reducing the life of the cell. Therefore, there is a problem that a sufficient amount of lithium difluorobis(oxalato)phosphate cannot be added.
A known approach to preventing swelling of a typical cell that uses an electrolyte containing a lithium salt is to provide an insulating inorganic oxide layer (hereinafter referred to as the insulating layer) on a separator as described in Japanese Patent Application Laid-Open No. 2009-199960 and Japanese Patent Application Laid-Open No. 2009-277597, for example.
The insulating layer provided on the separator inhibits decomposition of the electrolyte in the electrolyte solution near the insulating layer and therefore is believed to inhibit generation of gas.
However, while the insulating layers described in Japanese Patent Application Laid-Open No. 2009-199960 and Japanese Patent Application Laid-Open No. 2009-277597 can inhibit generation of gas during a cycle and storage, the effect of the insulating layers is not great enough to inhibit generation of gas during initial charge. Especially when lithium difluorobis(oxalato)phosphate is added to a nonaqueous electrolyte solution, it is difficult to adequately inhibit swelling that can be caused by gas generation during the initial charge.
That is, there has been a problem that a sufficient amount of lithium difluorobis(oxalato)phosphate cannot be added because lithium difluorobis(oxalato)phosphate generates gas during reductive decomposition and, if added to a nonaqueous electrolyte solution, swelling of the cell may be increased by gas generation after initial charge and discharge.